1. Field of the Invention
The present invention relates to an improved process for preparation of colored thermoplastic composite sheeting for use in laminated structures. More particularly, the invention relates to a solvent-free transfer process to provide uniformly colored thermoplastic composites wherein the colorants include finely divided crystalline light-stable pigments. The laminated structures in which the colored thermoplastic composites are incorporated are essentially transparent and exhibit improved shatter performance.
2. Brief Description of the Prior Art
It is known to provide a color image having a continuous gradation or a pattern in safety glass for automobiles and architectural applications. Techniques such as direct gravure printing processes have been used to print dye based images directly onto thermoplastic sheeting used as interlayers in glass laminates. However, these methods are not adaptable for use with inks containing non-diffusive pigments. The peaks and the valleys of the roughened surface of the sheeting are not uniformly coated. The peaks receive more coating than the valleys resulting in distinct dots when the printed thermoplastic sheet is incorporated in the laminated structure,
Other techniques such as solvent assisted transfer processes have been used to provide coloration of the thermoplastic sheeting. One such process is disclosed in published European Patent Application 0319583, published Jun. 14, 1989. In this process, an intermediate layer for laminated glass having a colored band containing dye and/or pigment is prepared by applying a volatile solvent to a plasticized polyvinyl butyral (PVB) sheet to dissolve PVB on the surface of the sheet to which a colored image is to be transferred, placing a colored image layer of a carrier substrate on top of the surface to which the solvent has been applied and then peeling away the carrier substrate. The composite structure so formed must then be dried to remove the solvent.
A process similar to the one described above utilizing a solvent to assist in the transfer of the colored image is disclosed in Japanese Patent Application 2-129049, published May 17, 1990. In this process, transfer of the colored image to the interlayer must be accomplished at temperatures of no more than 40.degree. C. in order to retain the roughness of the interlayer.
The use of elevated heat and pressure in the above-mentioned solvent assisted transfer processes disturbs the surface characteristics of the interlayer which is roughened during manufacture. Maintaining the roughened surface is essential during lamination to aid in the deaeration of the laminate. Incomplete deaeration results in the formation of bubbles and consequent poor adhesion of the interlayer to the substrate.
In addition to the adverse effect on surface characteristics, the use of solvent to facilitate transfer of the coating to the interlayer presents other problems. The colored interlayer after transfer must be dried to remove the solvent from the interlayer. This requires an additional drying step followed by a solvent recovery step to reduce solvent emmisions. Further, as disclosed in the above mentioned Japanese Patent Application 2-129049, drying of the solvent at elevated temperatures poses a danger of producing shrinkage and warping of the PVB sheeting. Complete removal of the solvent is essential to prevent formation of solvent bubbles and consequent poor adhesion of the interlayer in a glass or glass/plastic laminate structure.
It is therefore an object of this invention to provide a solvent-free transfer process to produce a composite comprised of a thermoplastic sheeting and a thin film containing colors or patterns. Another object is to provide a process whereby laminated structures having uniform, stable coloration is obtained without detracting from the physical properties of the structures. It is a further object of this invention to provide a process which can be carried out at accelerated speeds. Still another object of this invention is to provide laminated structures which exhibit improved shatter performance.